The present invention relates to an improved process for reducing the metals content of petroleum feedstocks, especially those feedstocks comprising high boiling fractions such as vacuum residua. The process comprises contacting the feedstock with phosphoric acid and separating a metals rich asphaltene fraction.
Demetallization using an acidic reagent has been suggested by various sources as one method to overcome the adverse effects of metallic contaminants found in petroleum feedstocks. For example, U.S. Pat. No. 2,778,777, granted Jan. 22, 1957, teaches the use of sulfuric acid to remove metals from petroleum oils. According to this patent the sulfuric acid reagent converts the metal containing constituents of the oil into water-soluble constituents or to constituents separable by conventional aqueous phase separation methods.
Subsequent investigators more thoroughly described the mechanism of acidic demetallization. They found that the metallic contaminants are innate constituents of the oil usually in the form of complex organic chelate compounds of the porphyrin type. Two forms of the porphyrin were observed, one volatile at temperatures between 1050.degree. F. and 1250.degree. F. and the other substantially non-volatile at such temperatures. The non-volatile contaminants are of low oil solubility and are for the most part colloidally dispersed in the oil. The volatile contaminants are soluble in oil and are for the most part in true solution.
U.S. Pat. Nos. 3,203,892, granted Aug. 31, 1965, and 3,245,902, granted Apr. 12, 1966, teach that when petroleum oils are treated with acidic reagents, such as concentrated sulfuric acid or halogenated acids, the principal reaction is a coagulation of the oil-insoluble metallic contaminants involving precipitation of the whole metallic molecule along with high molecular weight hydrocarbons associated with the metallic molecule. Thus, the coagulation step results in considerable loss of hydrocarbon material as acid sludge, and does not affect the volatile oil-soluble metallic contaminants. In order to remove these oil-soluble contaminants by acidic coagulation they must be converted to the oil-insoluble colloidal form by heat soaking or solvent precipitation. Thus, involving an extra processing step.
In order to overcome the deficiencies of acidic coagulation, U.S. Pat. Nos. 3,203,892 and 3,245,902 teach the use of hydrofluoric acid. It was found that, unlike other halogenated acids, hydrofluoric acid can be used to remove both oil-soluble and oil-insoluble metallic contaminants apparently by selective cleavage of the porphyrin molecule. The metals are removed as a solid insoluble filtrate while the bulk of the porphyrin molecule remains in the oil. Thus, the hydrocarbon content of the oil is essentially unaffected by this method of demetallization. However, hydrofluoric acid is a relatively hazardous material, and because of its highly corrosive properties is difficult to handle without special equipment.
Moreover, various conventional refining operations, especially residua processing, employ a preliminary deasphalting step. Thus, the use of an acidic coagulation to concentrate metallic contaminants in an asphaltene fraction would not appear to be a significant shortcoming where a deasphalting step was contemplated in any event. However, the acidic coagulation step also favors the formation of asphaltenes. Thus, demetallization methods which use acidic coagulation to concentrate the metallic contaminants in an asphaltene fraction and deasphalt the petroleum feedstock to remove the metals; unfortunately, result in an increased asphaltene content and a consequent loss of valuable hydrocarbon when the feedstock is deasphalted.
Accordingly, it is the primary object of the present invention to provide a process to remove metallic contaminants from petroleum feedstocks employing a relatively safe acidic reagent with a minimum formation of additional asphaltenes. This object and others are achieved by the invention summarized and then described in detail below.